Deciphering fungal dimorphism: Farnesol's unanswered questions.

Candida albicans excretes E,E-farnesol as a virulence factor and quorum sensing molecule that prevents the yeast to hyphal conversion. Polke et al. (2016) identified eed1Δ/Δ as the first farnesol hypersensitive mutant of C. albicans. eed1Δ/Δ also excretes 10X more farnesol and while able to form hyphae, it cannot maintain hyphae. This mutant enables new research into unanswered questions, including the existence of potential farnesol receptors and transporters, regulation of farnesol synthesis, and relationships among farnesol, germ tube formation and hyphal maintenance. The eed1 farnesol hypersensitivity can be explained by higher internal concentrations of farnesol or lower thresholds for response. One possibility invokes misexpression of a transporter. Saccharomyces cerevisiae and C. albicans have transporters for farnesylated peptides, like the a-factor pheromone, which could potentially also transport farnesol for virulence and quorum sensing. Significantly, these transporters are repressed in MTLa/MTLα C. albicans. An evolutionary pressure for C. albicans to become diploid could derive from its use of farnesol. Alternatively, maintenance of hyphal growth may increase the farnesol response threshold. Finally, Dpp1p, Dpp2p and Dpp3p are non-specific pyrophosphatases responsible for farnesol synthesis. Changes in expression of these enzymes do not explain differences in farnesol levels implicating involvement of additional factors like a scaffolding molecule.